Pneumatic conveying generally is utilized to transport dry, free-flowing, granular or pulverant material in suspension within a suitable conduit such as a pipe or duct by means of a high velocity airstream or by suction. A principal use of pneumatic conveying which has grown significantly in recent history is in the plastic processing industry wherein particulate such as plastic pellets or powdery resin is fluidized or suspended in a relatively contamination free airstream for transportation from bulk containers to a plurality of downstream plastic processing loaders.
With many prior pneumatic conveying systems, such as plastic material transporting systems, the primary material conduit is located upwardly of the receiving stations and branch conduits extend downwardly from the primary conduit and communicate with the material receivers. In such instances the primary conduit generally extends continuously across the juncture with the branch conduits and the selective material flow from the primary conduit to the respective receivers was often controlled by valves positioned in the primary conduit or the branch conduits adjacent the receivers.
The means of controlling material flow by the inclusion of a multiplicity of conduit valves is extremely expensive, requires constant maintenance and often results in clogging of the primary and/or branch transporting conduits. On this latter point it is noted that in the event of the cessation of vaccum by closing the valve of a downstream receiver for the purpose of directing the particulate stream to an upstream receiver, the inertia of the particulate in suspension will carry past the juncture of the primary conduit with the upstream receiver and settle and build up in the primary conduit at a location intermediate the upstream and downstream receiver. Such a settling and build up of particulate can result in subsequent rough material flow, decreased efficiency and, on occasion, complete system blockage. Furthermore, depending on exactly where the valves are located, or indeed if conduit valves are utilized, the particulate inertia, coupled with normal gravitational forces, may result in blockage of any one or more of the downwardly extending branch conduits.